Ultra-massive (⩾M⊙) oxygen/neon (ONe) core white dwarfs (WDs) are the result of the evolution of isolated progenitor stars with masses above 6−M⊙. It is expected that hydrogen-rich (DA) ultra-massive WDs harbor crystallized cores at the typical temperatures of the ZZ Ceti instability strip. These stars offer a unique opportunity to study the processes of crystallization and to infer their core chemical composition. We present a study of the evolution and asteroseismology of ultra-massive DA WDs. We found that all pulsating WDs known to date with M⩾1.1M⊙ should have more than 80% of their mass crystallized, if a ONe-core is assumed. Finally, we present a complete asteroseismological analysis to the well known ZZ Ceti BPM 37093 and a preliminary analysis to GD 518 and SDSS J0840+5222.